1. Field of the Invention
The present invention relates to a receiving apparatus.
2. Description of the Related Art
As a receiving apparatus in radio communication, a super-heterodyne system including an LO (Local Oscillator) 11 and a mixing unit 12 is generally known, as shown in FIG. 6, for example. In the receiving apparatus of the super-heterodyne system, a high-frequency RF (Radio Frequency) signal is mixed in the mixing unit 12 with a local oscillator signal from the LO 11 to be converted into an IF (Intermediate Frequency) signal so that a circuit handling a high frequency is not needed at a subsequent stage.
Furthermore, in the super-heterodyne system, since an RF signal is received which is referred to as an image signal having a frequency f2 (=2fL−f1) symmetric with a frequency f1 of the RF signal which is desirable to be received around a frequency fL of the local oscillator signal, an image cancelling mixer circuit having a function to remove the image signal is generally known as the mixing unit 12. In FIG. 3 of Japanese Patent Laid-Open Publication No. 2001-177425, there is disclosed an image cancelling mixer circuit where input high frequency signals are respectively mixed with a pair of local oscillator signals which are orthogonal in phase to each other and a generated pair of mixed signals is used.
As described above, by removing the image signal causing interference in the super-heterodyne system, only the RF signal having a desired frequency can be received.
In the above image cancelling mixer circuit, there is a problem that phase shift or gain shift is caused by variation in components to be used or change in ambient temperature, and when the generated pair of mixed signals are not orthogonal in phase to each other or when the signals are different in amplitude, the image signal cannot be completely removed. Therefore, in the image cancelling mixer circuit disclosed in FIGS. 10, 13, 16, etc., in Japanese Patent Laid-Open Publication No. 2001-177425, the phase shift and the gain shift are corrected using an all-pass filter, a delay circuit, and a gain control circuit. However, in the correcting method, in order to reliably remove the image signal, it is required to grasp frequency characteristics, temperature characteristics, voltage characteristics, etc., of the all-pass filter or the like and to change a method of controlling the all-pass filter or the like according to each of the characteristics.
Therefore, a process is required in which the image cancelling mixer circuit is adjusted in advance in every receiving apparatus.